Concurrency Fixes (#283)

* Review places where MainActor used

* Adapt to MainActor changes, avoid redundant task detachments

* Move Rust client operations into a dedicated concurrent queue, make init async

* Add changelog

* Fixed small working for awaiting non-async method

* Remove some redundant tasks

* Put back some main actors

* Fix tests

* Fixed timeline updates when the number of items doesn't change. Keeps more previous home screen room data between reloads

Co-authored-by: Stefan Ceriu <stefanc@matrix.org>

UnitTests/Sources/BackgroundTaskTests.swift

@@ -18,6 +18,7 @@ import XCTest
 
 @testable import ElementX
 
+@MainActor
 class BackgroundTaskTests: XCTestCase {
     private enum Constants {
         static let bgTaskName = "test"